Avoid displaying -ve amounts for availability.
Treat JetFuel < 0.01u as no JetFuel.

There aren't any tanks that small (that I know of), and 50cc is good for
maybe a model airplane: any discrepancy that small is likely due to
precision losses.
Widen the resource amount boxes.

Now they won't wrap with 2 decimal places for quantities in the 1000s, and
hopefully even 10000s.
Merge pull request #40 from taniwha-qf/gui-tweaks

Gui tweaks